Certain polyol fatty acid polyesters have been suggested as low or reduced calorie substitutes for triglyceride fats and oils used in foods. For example, nonabsorbable, nondigestible sugar fatty acid esters or sugar alcohol fatty acid esters having at least 4 fatty acid esters groups with each fatty acid having from 8 to 22 carbon atoms have been used as partial or full fat replacers in low calorie food compositions. See Mattson and Volpenhein; U.S. Pat. No. 3,600,186; Issued Aug. 17, 1971. Likewise, certain intermediate melting polyol polyesters have been developed that provide passive oil loss control, while at the same time reducing waxiness in the mouth. See Bernhardt; European Patent Application Nos. 236,288 and 233,856; Published September 9, and Aug. 26, 1987, respectively. Blends of completely liquid polyol polyesters with completely solid polyol polyester hardstocks, preferably esterified with C.sub.10 to C.sub.22 saturated fatty acids (e.g. sucrose octastearate) have also been proposed in order to provide passive oil loss control. See for example, Jandacek; U.S. Pat. No. 4,005,195; and Jandacek/Mattson; U.S. Pat. No. 4,005,196; Both issued Jan. 25, 1977.
A number of different processes have been disclosed in the art for preparing highly esterified polyol fatty acid polyesters, in particular sucrose polyesters, useful as reduced calorie fat substitutes. One such process for preparing these polyesters involves a solvent-free, essentially two-step transesterification of the polyol (e.g., sucrose) with the fatty acid esters of an easily removable alcohol (e.g., fatty acid methyl esters). In the first step, a mixture of sucrose, methyl esters, alkali metal fatty acid soap and a basic esterification catalyst are heated to form a melt. The amount of methyl esters is such that the melt forms primarily partial fatty acid esters of sucrose, e.g., sucrose mono-, di- and/or triesters. In the second step, an excess of methyl esters are added to this melt which is then heated to convert the partial sucrose esters to more highly esterified sucrose polyesters, e.g., sucrose hexa-, hepta-, and particularly octaesters. See, for example, U.S. Pat. No. 3,963,699 (Rizzi et al.), issued Jun. 15, 1976; U.S. Pat. No. 4,517,360 (Volpenhein), issued May 14, 1985; and U.S. Pat. No. 4,518,772 (Volpenhein), issued May 21, 1985, which disclose solvent-free, two-step transesterification processes for preparing highly esterified polyol fatty acid polyesters, in particular highly esterified sucrose polyesters.
In some processes for preparing highly esterified polyol fatty acid polyesters, all of the fatty acid methyl esters are added to the polyol (e.g., sucrose) at the beginning of the reaction, i.e. a one-step addition process. See, for example, U.S. Pat. No. 4,611,055 (Yamamoto et al.), issued Sep. 9, 1986. Like the previously described two-step processes, such one-step processes first form primarily partial fatty acid esters of sucrose that are then converted to more highly esterified sucrose polyesters. Accordingly, these single-step and two-step processes are collectively referred to hereinafter as "two-stage" transesterifications, wherein the "first stage" involves the formation of the partial esters and wherein the "second stage" involves the conversion of the partial esters to more highly esterified polyesters.
Alternatively, highly esterified polyol polyesters may be prepared by two stage solvent-based processes, (see, for example, U.S. Pat. No. 4,954,621 (Masaoka et al.), or one stage solvent-based or solvent free processes, see for example, U.S. Pat. No. 4,968,791, (Van Der Plank), issued Nov. 6, 1990; U.S. Pat. No. 5,079,355 (Meszaros Grechke et al.) issued Jan. 7, 1992; or U.S. Pat. No. 5,071,975 (Ver der Plank et al.) issued Dec. 10, 1991.
The methyl esters which are used to prepare the polyol polyesters can be prepared by the transesterification of triglyceride oils and fats with methanol in the presence of an alkaline catalyst. After the transesterification reaction, a crude glycerine layer comprising glycerol formed in the transesterification reaction, soap formed by the catalyst, catalyst, some methyl esters and methanol, is separated from the fatty-acid methyl ester layer. The methyl ester layer is then purified by any suitable recovery method, such as e.g. distillation. Processes of this type have been described in U.S. Pat. Nos. 2,383,596, 2,383,579, 2,383,580, 2,383,596, 2,383,599 2,383,601, 2,383,602, 2,383,614, 2,383,632 and 2,383,633 and in the European Pat. No. 0 164 643. An extra esterification step before recovery, but after separation of the fatty acid methyl ester layer from the glycerol layer may optionally be used to produce high yields of high purity fatty acid methyl esters. See European Pat. No. 391 485.
Unfortunately, the methyl esters prepared by any of these known processes are likely to contain some residual level of fat sources such as glycerine, and mono-, di-, or triglyceride. When these fat-containing methyl esters are used to prepare polyol fatty acid polyesters, they will cause the polyol polyester product to contain undesirably high levels of triglyceride fat. Although the triglyceride fat is typically present in the polyol polyesters at levels below 2%, these triglycerides nevertheless add calories to the polyol polyester and keep the polyol fatty acid polyesters from being completely fat-free. It is, therefore, an object of the present invention to prepare methyl esters containing minimal levels of glycerine and mono-, di-, and triglyceride for use in preparing polyol polyesters having a triglyceride level of less than 0.5%.
Another disadvantage with known processes for preparing methyl esters is that on a production scale an excessively high level of residue is formed on the bottom of the still or distillation apparatus during ester distillation. Typically, glycerine levels of 1 to 11/2% are present in methyl esters even after esterification and gravity decanting of the glycerine layer as a result of difficulty in coalescing substantially all of the glycerine in a production scale settling tank. If glycerine is not effectively separated from the methyl esters either through centrifugation, extraction or absorption, before heat treatment and/or distillation, substantial levels of di-glycerides and tri-glycerides will form, possibly in excess of 10% of the methyl ester. Di-glycerides and tri-glycerides are not volatile and remain in the residue at the bottom of the still.
It is, therefore, another object of this invention to ensure that a minimal level of glycerine is present in the methyl ester prior to the distillation in order to minimize the amount of residue during the distillation to less than 10%, and preferably less than 5%. This offers advantages in minimizing still bottom recycle streams as well as maximizing finished product yields in ester-making processes.